Primary Studies On Physiological And Molecular Mechanism Of Sugar Metabolism In Potted Poinsettia

Poinsettia (Euphorbia pulcherrima willd.) is one of the staple potted flowers at home and abroad, however, the most important ornamental organ in poinsettia isn't its flower but its bract, a kind of red abnormal leaves on the top of the plant. Commodity value of potted Poinsettia was affected mainly by the size and color of bract. The growth and development as well as anthocyanin biosynthesis in bract based on sugar content. So far, studies on physiological mechanism of sugar metabolism were few during the growth and development of bract.Using'Early Velvet'as materials, the net photosynthetic rate, pigment content, sugar content and activities of sucrose-metabolizing enzymes were tested on pale green leaves (transition leaves) and bracts on the top of plants during the three stages of bract growth and development. An acid cell wall invertase gene fragment was cloned by PCR, expressed by RT-PCR and Real-time PCR in the roots, stems, leaves and buds of poinsettia, as well as in the green leaves and transition leaves from green to red during the growth and development of bract. The results were as follows:1. The chlorophyll content and net photosynthetic rate in the upper pale green leaves on October 31 were less than those in the lower green leaves, which suggested that the upper pale green leaves could partly photosynthesis by themselves but rely mainly on the heterotrophic supplies. However, the upper red leaves developed from bract in November contained principally anthocyanins and no more chlorophylls, as well as negative net photosynthetic rate, which suggested that the growth and development of bract relied wholly on the heterotrophic supplies.2. The activities of AI and NI increased, especially AI activities increasing by one times, and meanwhile the SS synthesis activities and SPS activities all dropped in red bracts during their rapid growth compared with those in pale green leaves on October 31, which suggested that bracts might enhance the sink strength to promote sucrose shifting from stems to leaves by decreasing the activites of sucrose synthetic enzymes and increaseing the activities of sucrose cleavage enzymes. Contrarily, the activities of SS synthetic enzymes and SPS rose, accompanied by the decrease of the activities of AI and NI, which resulted in sucrose accumulation in green leaves, to heighten sucrose gradient between leaves and stems and beneficial for transition of sugar from leaves to stems. After red bract developed, the activities of AI, NI, SS and SPS all dropped, which would reduce sugar accumulation in bracts and increase sugar accumulation in the stems and the other organs to supply bracts with sugar for long time.3. One invertase gene fragment containing a catalytic cysteine isolated from genomic DNA of Poinsettia by PCR. The fragment has 523bp, encodes 173 amino acids, belongs to an open reading frame without intron. It has highly homologous with the other plants acid cell wall invertase genes through homologous comparison. So it was speculated that the fragment belongs to one of acid cell wall invertase genes.4. The relative expression levels of Poinsettia acid cell wall invertase gene were analysised by real-time PCR. It was found that the gene expression levels were the strongest in roots, general in leaves and the weakest in stems, among them, the EpCWINV expression levels in roots were 4397 times with that in stems. During bract development, the expression levels of EpCWINV all rose in green leaves and transition leaves, but dropped in their petioles, which suggested that the EpCWINV might play different roles in leaves and petioles.